1. Field of Invention
The present invention relates to a height adjustment structure. More particularly, the present invention relates to a height adjustment structure inside a scanning device for adjusting distance of separation between an optical system and a document holder panel.
2. Description of Related Art
As a result of the rapid advances in multimedia software, network connection and computer technologies, the scanner has become an indispensable peripheral device for a computer system. In general, a platform-type scanner includes an optical system and a driving device. Through the driving device, the chassis housing the optical system is driven so that a sensor in the optical system may scan the image on a document in steps and turn the scanned image into electronic signals. Normally, the optical sensor is a charge-coupled device (CCD).
FIG. 1 is a perspective view showing a guiding rod, a driving device and an optical system in a conventional platform-type scanner. FIG. 2 is a side view of the document holder, the guiding rod and the optical system in the conventional platform-type scanner. As shown in FIG. 1, a light source 12 and a slit 14 are positioned over the optical system 10. Light from the light source 12 penetrates a transparent document panel 40 to arrive at the surface of a document 50. After reflection, the reflected light passes through the slit 14 and enters the optical system 10 for image processing. By driving transmission device 20 forward slowly, the optical system 10 advances in steps until the entire document 50 is scanned.
Since the optical system 10 must advance in a preset direction linearly with precision to avoid image distortion, the optical system 10 needs to follow a guiding rod 30. Hence, the chassis that houses the optical system has a positioning hole 16 for sliding along the guiding rod 30. As shown in FIG. 2, the transmission device 20 drives one side of the chassis. Therefore, a roller 18 is installed on the other side of the chassis so that the document 50 on the panel 40 can move in parallel and prevent any tilting of the optical system 10.
In general, an optical system includes a light source, a reflecting mirror, transparent lenses and an optical sensor. All these components are housed inside a chassis. Hence, for similar design, the lens inside the optical system must have a fixed object distance. However, for different types of platform scanner design, distance 42 from the document panel 40 to the guiding rod 30 may be slightly different. Consequently, distance 44 from the optical system 10 to the document panel 40 needs to be adjusted so that the document 50 is in proper focus with respect to the object distance of the lens system (not shown) inside the optical system 10. In other words, the distance from the document panel 40 to the lens inside the optical system 10 must be properly set in each design. Since the distance must be carefully set for each scanner design, production and design cost are greatly increased.